The present invention relates to a digital-to-analog converter used in a complementary metal-oxide semiconductor (CMOS) image sensor circuit; and, more particularly, to a digital-to-analog converter that makes the amplification rate of a pixel signal change linearly according to digital control codes inputted thereto.
Generally, an image sensor is a device that captures images by using a photo-reactive characteristic of a semiconductor. With different brightness and wavelength, every aspect of a subject existing in nature shows a different electrical value at each pixel of a sensing device. It is the image sensor that converts the electrical values into signals that can be processed.
Recently, image sensors have received additional attention as they are applied to a variety of security equipment, video conference cameras, digital still cameras, PC cameras, next-generation PDAs, with a function of transmitting image information and other data.
Image sensors are classified into two types: One type is a charge-coupled device (CCD) image sensor; and the other type is a complementary metal-oxide semiconductor (CMOS) image sensor. Compared to a CCD image sensor, a CMOS image sensor can be operated more easily and implement a variety of scanning methods. Also, because a CMOS image sensor is capable of integrating a single processing circuit in one chip, the CMOS image sensor may assist in miniaturizing products as well as reducing production costs with its CMOS technology. Additionally, with remarkably low power consumption when compared to the CCD, the CMOs image sensor""s applicable fields continue to expand.
Conventionally, a CMOS image sensor comprises a comparator for comparing a ramp signal, as a reference signal, which decrease regularly, and an analog data signal from a photodiode; a counter to initiate counting when the ramp signal is outputted; and a latch for storing a counted value in a digital data value according to the value of the comparison result.
To perform the operation described above, the CMOS image sensor includes a digital control block for outputting various control signals. The digital control block sets up a range of storable digital data values, for example, from 0 to 255, according to the brightness of an image, by inputting a digital control code with a digital-to-analog converter. A digital control code is a control code a user inputs arbitrarily that controls the range of storable digital data values according to the brightness of an external light.
The CMOS image sensor equips the digital-to-analog converter at the digital control block so as to set up the range of digital data values (e.g. 0-255). According to the output voltage from the digital-to-analog converter, the unit voltage of a ramp signal according to the reference clock is determined.
For example, when an input digital control code is binary xe2x80x9800001xe2x80x99, the range of storable digital data values is binary xe2x80x9800000000-11111111xe2x80x99 (0-255). If the input digital control code is binary xe2x80x9800010xe2x80x99, the range of digital data values is binary xe2x80x9800000000-01111111xe2x80x99 (0-127). Similarly, if the input digital control code is binary xe2x80x9800100xe2x80x99, the range of digital data values becomes binary xe2x80x9800000000-00111111xe2x80x99 (0-63). When the input digital control code is binary xe2x80x9801000xe2x80x99, the range becomes binary xe2x80x9800000000-00011111xe2x80x99 (0-31).
The bigger the range of digital data values is, the brighter the image is stored. When a digital data value is 255, the screen becomes about twice as bright as when the digital data value is 127. Therefore, if a user wants to see a dark part brighter, the range of storable digital data values should be increased by making the value of a digital control code smaller.
FIG. 1 is a circuit diagram illustrating a digital-to-analog converter of a conventional CMOS image sensor, which is implemented in three bits.
Referring to FIG. 1, the digital-to-analog converter comprises a voltage divider 10 for equally distributing a reference voltage; a switching block 20 for switching the distributed resistances to an output block; and an output block 30 for outputting signals through the switching block 20.
The voltage divider 10 is composed of eight resistors (R) serially arrayed between the reference voltage (Vref) and the ground voltage level. The switching block 20 is comprised of a plurality of N channel MOS transistors configured so that only one path should be selected by a decoded digital value. The output block 30 is comprised of an operation amplifier that feedbacks the output to a negative (xe2x88x92) input and thus serves as a buffer. When connected as shown in FIG. 1, an operation amplifier operates as a buffer with a gain of 1.
Referring to FIG. 1, the operation of a digital-to-analog converter will be described below.
When the reference voltage (Vref) from the circuit shown in FIG. 1 is xe2x80x981Vxe2x80x99 and the digital control code xe2x80x98001xe2x80x99 is inputted into the switching block 20, switches receiving the signals /b1, /b2, /b3 inputted to the switching block 20 are turned on, and a voltage of xe2x85x9V is output to the output block 30. Also, when the digital control code is increased from xe2x80x980001xe2x80x99 to xe2x80x980100xe2x80x99, the output values of the digital-to-analog converter increase to xe2x85x9V, {fraction (2/8)}V, xe2x85x9cV and {fraction (4/8)}V, respectively. That is, as the resistance values of the digital-to-analog converter is regular, the output voltage differences increase regularly as well according to input digital control codes.
A conventional digital-to-analog converter of a CMOS image sensor uses an array of 32 resistors and outputs 32 voltage values. The voltage values determine the unit voltage of a ramp signal input to the comparator of the CMOS image sensor. A ramp signal decreases as much as the unit voltage, e.g., {fraction (1/256)}, each clock pulse, so the size of the unit voltage is determined by the output value of the digital-to-analog converter before the output value is input to the comparator.
For instance, when the digital control code is binary xe2x80x9800001xe2x80x99, the output voltage of the digital-to-analog converter is {fraction (1/32)}V. Then, from the output value, the unit voltage of a ramp signal {fraction (1/256)}V is output. If the digital control code is binary xe2x80x9800010xe2x80x99, the voltage output from the digital-to-analog converter is {fraction (2/32)}V, and from this voltage, the unit voltage of a ramp signal is obtained to be {fraction (2/256)}V. After that, when the digital control code increases to xe2x80x9800011xe2x80x99 and xe2x80x9800100xe2x80x99, the unit voltages of the ramp signal increases to {fraction (3/256)}V and {fraction (4/256)}V, respectively, in the CMOS image sensor.
The unit voltage of a ramp signal is an amount of decreased voltage per clock cycle. As the unit voltage becomes smaller and smaller, the ramp signal decreases for many clock cycles, and as the unit voltage becomes bigger and bigger, the ramp signal decreases for fewer clock cycles. The number of clock cycles used while the ramp signals are supplied becomes the range of storable digital data values in the image sensor. That is, if the digital control code is 1000011, the ramp signal of the comparator decreases {fraction (1/256)}V per clock cycle, and thereafter, the ramp signal decreases for a total of 255 clock cycles. This means that the range of storable digital data values is from 0 to 255.
FIG. 2A is a schematic diagram showing a ramp signal of a CMOS image sensor wherein the digital control code input to the digital-to-analog converter is binary xe2x80x9800001xe2x80x99.
Referring to FIG. 2A, it is shown that the ramp signal decreases as much as the unit voltage, i.e., {fraction (1/256)}V, per clock cycle when a digital control code is xe2x80x9800001xe2x80x99. Therefore, the range of digital data values that can be stored in the CMOS image sensor is xe2x80x98from 0 to 255xe2x80x99.
FIG. 2B is a schematic diagram showing a ramp signal of a CMOS image sensor, wherein the digital control code inputted to the digital-to-analog converter is binary xe2x80x9800010xe2x80x99.
Referring to FIG. 2B, when the digital control code is xe2x80x9800010xe2x80x99, the ramp signal decreases as much as {fraction (2/256)}V per clock cycle, i.e., the unit voltage, and the range of digital data values storable in the CMOS image sensor is xe2x80x98from 0 to 127xe2x80x99. Also, if the digital control code is xe2x80x9800011xe2x80x99, the range of digital data value is xe2x80x98from 0 to 63xe2x80x99. As described above, the wider the range of the storable digital values becomes, the brighter the image information is stored.
FIG. 3 is a graph showing the range of the storable digital data values to a digital control code, which is input to the digital-to-analog converter in the CMOS image sensor, and it is represented in TABLE 1 below.
Referring to TABLE 1, even if a digital control code is increased gradually, the range of storable digital data values is not gradually decreased. From this, we can see that the brightness of a stored image of a subject does not change linearly, even if a digital control code is changed linearly. In addition, the brightness of an image cannot be controlled delicately, even though the digital control code is controlled to store the image brighter in a dark place.
Of course, it is possible to control the digital control code and control the brightness of a stored image rather delicately, when the digital control code is relatively big, such as 00110. However, fine controlling here is meaningless and it""s still hard to control the range of storable digital data values finely to change the brightness of an image in a practical area, e.g., when the digital control code ranges from 0001 to 0100.
Therefore, a CMOS image sensor is needed that can control the brightness of a stored image by making linear relations between the range of digital data values and the digital control codes.
In accordance with an embodiment of the present invention, there is provided a digital-to-analog converter, comprising: an array of resistors with weights on their resistance value, for distributing a voltage delicately and outputting a plurality of analog signals; a switching block for selecting and outputting one of the outputs of the arrayed resistors according to digital control codes; and an output means for buffering and outputting an output from the switching block.
In accordance with an embodiment of the present invention, there is provided a CMOS image sensor, comprising: a ramp signal control block for outputting a ramp signal that decreases as much as a unit voltage regularly per clock cycle; and a digital-to-analog converter that outputs a reference voltage to determine the size of the unit voltage. The digital-to-analog converter comprises: a) an array of resistors with weights on their resistance value, for distributing a voltage delicately and outputting a plurality of analog signals; b) a switching block for selecting and outputting one of the outputs of the arrayed resistors according to digital control codes; and c) an output means for buffering and outputting an output from the switching block.